1. Field of the Invention
The present invention relates to a method of manufacturing a negative electrode for a nonaqueous electrolytic rechargeable battery that reduces irreversible capacity of the negative electrode and prevents the capacity of the nonaqueous electrolytic rechargeable battery from reducing, and the nonaqueous electrolytic rechargeable battery using it.
2. Background Art
As the number of portable and cordless electronic apparatuses has been recently increased, expectations on small and light nonaqueous electrolytic rechargeable batteries having high energy density have been raised. Presently, carbon material such as graphite has been in practical use as negative electrode active material of the nonaqueous electrolytic rechargeable batteries. However, the theoretical capacity density of the carbon material is 372 mAh/g (833 mAh/cm3). For increasing the energy density of the nonaqueous electrolytic rechargeable batteries, silicon (Si), tin (Sn) or germanium (Ge) that is combined with lithium to form an alloy, oxide thereof, and alloy thereof are studied. The theoretical capacity densities of these negative electrode active materials exceed 833 mAh/cm3, and are larger than that of the carbon material. Of these negative electrode active materials, silicon-containing particles such as Si particles and silicon oxide particles are inexpensive and hence are studied widely.
When carbon material or negative electrode active material of which theoretical capacity density exceeds 833 mAh/cm3 is used in a nonaqueous electrolytic rechargeable battery, however, the irreversible capacity is generally large and hence a nonaqueous electrolytic rechargeable battery having a large battery capacity cannot be obtained.
The irreversible capacity means a capacity eliminated by initial charge and discharge. In a charge/discharge reaction of a conventional nonaqueous electrolytic rechargeable battery, lithium discharged from the positive electrode is stored in the negative electrode during charge, and lithium discharged from the negative electrode is stored in the positive electrode during discharge. When the irreversible capacity in the negative electrode is large at the time of initial charge and discharge, part of the reversible capacity originally possessed by the negative electrode and positive electrode runs down, and hence a nonaqueous electrolytic rechargeable battery of large battery capacity is not obtained. The irreversible capacity of the negative electrode is caused by partial inactivation of lithium that is caused by side reaction with electrolyte occurring during charge or by retention of reversible lithium that is caused by hysteresis of storage/discharge voltage of lithium. The irreversible capacity is thought to cause the reduction of available reversible capacity.
For reducing the decrease in battery capacity caused by the irreversible capacity of the negative electrode, technology of previously charging lithium into the negative electrode is proposed.
For example, Japanese Patent Unexamined Publication No. 2005-38720 (hereinafter referred to as “patent document 1”) discloses the following method of manufacturing a negative electrode. A light metal layer made of metal lithium or the like is formed on a negative electrode active material layer containing a polymeric binder by a dry film forming method such as a direct vacuum deposition method, and is then stored in a dry atmosphere or electrolyte. Thus, lithium is previously stored in the negative electrode.
International Publication No. 96/27910 (hereinafter referred to as “patent document 2”) discloses another manufacturing method. In this method, metal lithium foil or the like is stuck by roll transfer or the like to a negative electrode sheet having a composite oxide containing tin or the like, a battery is then formed, and electrolyte is injected into the battery, thereby previously storing lithium in the negative electrode.
In the negative electrode in patent document 1, since vaporized metal lithium of 500° C. or higher is directly formed on the negative electrode active material layer by the vacuum deposition method, the polymeric binder and the negative electrode active material in the negative electrode active material layer are deteriorated by solidification heat of metal lithium during film forming. As a result, the function as a binder decreases in the polymeric binder. In the negative electrode active material, crystallization or the like of amorphous material in the negative electrode active material reduces the reversibility of lithium ions, and hence can reduce the charge/discharge cycle characteristic.
In the negative electrode in patent document 2, since the metal lithium foil is stuck to the whole surface of the negative electrode sheet, the thickness of the metal lithium foil must be 30 μm or smaller in consideration of the amount of lithium stored in the negative electrode.
When the metal lithium foil is thin, however, it is extremely difficult to manufacture, manage, and handle it from the view point of mechanical strength, surface adherence, and safety. Therefore, high productivity cannot be obtained.
For preparing metal lithium foil having a handleable thickness, a method of partly sticking metal lithium foil onto the negative electrode active material layer in a strip shape is considered, for example. Even in this case, since the metal lithium foil is stuck onto the negative electrode active material layer in the strip shape, the storage amount of metal lithium in the negative electrode active material layer depends on the existence of the metal lithium foil. As a result, the negative electrode partly deforms due to expansion, or nonuniform reaction occurs during charge and discharge.
In the conventional metal lithium foil produced by extrusion molding or rolling, the surface is not flat and variation in thickness cannot be controlled to be 5 μm or smaller. Therefore, it is difficult to uniformly stick the metal lithium foil to the negative electrode active material layer. As a result, disadvantageously, variation in battery capacity is large, and controlling the variation reduces the productivity significantly.